Implements mail transport using the external programs 'mailx', Mail, or 'mail'. When instantiated, the mailer will look for any of these binaries in specific system directories, and the first program found is taken.

WARNING: There are many security issues with mail and mailx. DO NOT USE these commands to send messages which contains data derived from any external source!!!

Under Linux, freebsd, and bsdos the mail, Mail, and mailx names are just links to the same binary. The implementation is very primitive, pre-MIME standard, what may cause many headers to be lost. For these platforms (and probably for other platforms as well), you can better not use this transport mechanism.

There are two version of the mail program. The newest accepts RFC822 messages, and automagically collect information about where the message is to be send to. The BSD style mail command predates MIME, and expects lines which start with a '~' (tilde) to specify destinations and such. This field is autodetect, however on some platforms both versions of mail can live (like various Linux distributions).

Fatal error: the specific package (or one of its superclasses) does not implement this method where it should. This message means that some other related classes do implement this method however the class at hand does not. Probably you should investigate this and probably inform the author of the package.

The message which is sent is the result of a bounce (for instance created with Mail::Message::bounce()), and therefore starts with a Received header field. With the bounce, the new destination(s) of the message are given, which should be included as Resent-To, Resent-Cc, and Resent-Bcc.

The To, Cc, and Bcc header information is only used if no Received was found. That seems to be the best explanation of the RFC.

As alternative, you may also specify the to option to some of the senders (for instance Mail::Transport::SMTP::send(to) to overrule any information found in the message itself about the destination.